This invention relates to tool holders, and, more particularly, to a tool holder having an improved insert clamping mechanism and a high velocity coolant delivery system for breaking chips and extending tool life.
Tool holders for performing metal working operations such as turning, boring, shaping, grooving and threading generally comprise a support bar formed with a cavity which is adapted to receive a shim in the form of a flat plate, or a support block formed with a seat. An insert having a top surface terminating with a cutting edge is mounted atop the shim, or within the seat in the support block, so that the top surface of the insert is exposed and the cutting edge extends outwardly from the holder. In some insert designs, particularly for threading operations, the top surface of the insert is formed with a V or U-shaped groove or notch formed at an angle relative to the cutting edge.
The insert is secured atop the shim, or within the seat of the support block, by a clamp which comprises an arm having one end terminating with a clamping edge. The clamp is bolted to the support bar so that the clamping edge engages the top surface of the insert forming essentially line contact therebetween.
One problem with the clamping mechanism of prior art tool holders is that the clamping force applied by the clamps described above is limited to the line of contact between the top surface of the insert and the clamping edge of the clamp. Additionally, in clamping most inserts, the clamp applies a force in only one direction against the insert urging it downwardly atop the shim or the bottom surface of the seat formed in the support block. As a result of the application of a clamping force to the insert over a limited area and in a single direction, the insert often becomes loosened within the tool holder cavity during a cutting operation which quickly results in failure of the insert.
Some improvement in clamping is achieved in prior art tool holders by employing the notched type of insert described above. The clamping edge of the clamp engages the sidewall of the U-shaped notch so as to urge the insert downwardly, and, to a limited degree, inwardly within the seat in the tool holder due to the angle of the notch. The problem with this design is that relatively close tolerances must be maintained in forming the notch in the insert, and in positioning the clamp and the tool holder, so that the clamping edge of the clamp engages the sidewall of the notch in the top surface of the insert. If such tolerances are not maintained therebetween, the insert cannot be securely clamped within the seat and becomes loosened during a machining operation.
In performing machining operations with prior art tool holders such as described above, the cutting edge of the insert is moved into engagement with a workpiece to remove a chip of metal. The chips comprise a plurality of thin, generally rectangular shaped sections of metal which slide relative to one another along shear planes when separated from the workpiece. This shearing movement of the thin metal sections forming the chip generates a substantial amount of heat in addition to the heat generated by abrasion of the cutting edge of the insert as it contacts the workpiece.
Among the causes of failure of the cutting inserts mounted in the types of tool holders employed in the prior art are abrasion between the cutting insert and workpiece, and a problem known as cratering. Cratering results from the intense heat developed in the formation of the chip, and the frictional engagement of the chip with the cutting insert. As the metal forming the chip is sheared from the workpiece, it moves along the top surface of the insert and in some cases along the clamp which secures the insert in place. Many inserts include a chip breaker groove on the surface which faces the chip for turning the chip upwardly away from the top surface of the insert. However, even with chip breaker grooves, at least a portion of the upper surface of the insert inwardly from its cutting edge is in frictional engagement with the chip.
Due to this frictional engagement, and the intense heat generated in the formation of the chip, craters are formed on the exposed, upper surface of the insert. Once these craters become deep enough, the entire insert is subject to cracking and failure along its cutting edge, and along the sides of the insert. Cratering has become a particular problem in recent years due to the development and extensive use of alloy steels, super hard alloys such as titanium, stainless and nickel-based alloys.
One approach to obtaining increased insert life has been to attempt to reduce the temperature of the cutting insert and chip by a quenching operation in which the tool holder and workpiece are flooded with a low pressure stream of coolant consisting of a mixture of oil and water. Typically, a nozzle is disposed several inches above the cutting tools and workpiece which directs a low pressure stream of coolant onto the workpiece tool holder and on top of the chips being produced. This technique, known as flood cooling, effectively cools only the upper surface of the chips and that portion of the tool holder near the edge of the cavity wherein the insert is clamped.
One limitation of prior art tool holders is that most are designed for use with flood cooling systems of the type described above. However, flood cooling has proven to be ineffective in removing heat from the cutting area. The underside of the chip which makes contact with the cutting insert, and the interface between the cutting insert and workpiece, are not cooled by a low pressure stream of coolant directed from above the tool holder. This is because the heat produced in the area of the chip and the cutting edge of the insert, particularly at the high operating speeds of modern milling or turning machines, vaporizes the coolant well before it can flow near the cutting edge of the insert.
Prior art tool holder have been designed to improve upon the flood cooling technique of directing a low velocity stream of coolant onto the cutting area from a location above the tool holder. Such tool holders include coolant delivery passageways terminating with one or more discharge orifices oriented to eject coolant across the top surface of the insert and beneath the chips being formed. The designs known to the inventor fail to achieve cooling in the immediate area of the cutting edge - workpiece interface where the intense heat is produced, and are thus no more effective than the other flood cooling techniques described above.
Tool holders of the types described above, which employ flood cooling, are not only ineffective in prolonging insert life but can actually reduce insert life in some instances due to thermal failure of the inserts. This occurs because a high temperature gradient is developed between the very hot area immediately surrounding the cutting edge of the insert, and the cooler inner portion of the insert mounted in the cavity of the tool holder. The coolant cannot reach the cutting edge of the insert before it is vaporized and thus effectively cools only the area of the insert which is held in the tool holder. This extreme difference in temperature between the cutting edge and the remainder of the cutting insert can result in thermal failure.
In addition to limited tool life, another pervasive problem with tool holders currently employed in the cutting tool industry involves the proper breakage and removal of chips from the area of the cutting insert and holder. Preferably, chips should be broken into short segments when sheared from the workpiece. If they are not broken but form in a continuous length, the chips tend to wrap around the cutting insert, tool holder and/or the workpiece which can lead to tool failure or at least require periodic interruption of the machining operation to clear the area of impacted or bundled chips.
Current attempts to solve the chip breaking and removal problem are limited to various designs of cutting inserts having a chip breaker groove, which is a groove formed in the top surface of the insert immediately adjacent the cutting edge. Chip breaker grooves engage the chips as they shear from the workpiece and turn or bend them upwardly from the surface of the insert so that they tend to fracture. While acceptable performance has been achieved with some chip breaker groove designs in some applications, variables in machining operations such as differing materials, types of machines, depths of cuts, feed rates and speeds make it virtually impossible for one chip breaker groove design to be effective in all applications. This is evidenced by the multitude of chip breaker designs now available which are intended to accommodate the widely varying machining conditions which can occur in industry. Selection of a suitable cutting insert for a particular application, if one is available at all, can be a difficult problem.